Measuring system



Dec. 5, 1967 v R. H. COTHER 3,356,868

MEASURING SYSTEM l l INVENT nef/@r A4 (aff/f Dec. 5, 1967 R. H. COTHER 43,356,868

Y MEASURING SYSTEM l Original Filed July 29, 1963 2 Sheets-Sheet 2United safes Patent YQ ration of California Continuation of applicationSer. No. 298,125, July 29, 1963. This application Feb. 15, 1967, Ser.No. 616,408

6 Claims. (Cl. 310-S.4)

This application is a continuation of Ser. No. 298,125, filed July 29,1964, now abandoned.

This invention relates to measuring systems and more particularly toimprovements in systems for amplifying the output of charge-generatingsources such as piezoelectric transducers.

Though this invention may be employed in connection with other types ofcharge-generating sources, its most important applications now knownmake use of piezoelectric transducers that are employed for detectingvariable forces or motions. An important application of the inventioninvolves the detection and measurement of accelerations by means of apiezoelectric transducer. For this reason, to facilitate anunderstanding of the invention, it will be described herein withspecific reference to systems that utilize piezoelectric accelerometers.

It is common to employ piezoelectric accelerometers to study the motionof vibrating objects. The vibrations that are usually studied ofteninvolve components having frequencies that extend over a wide range. Inmany such applications, it is desirable to measure accelerationsaccurately over a frequency range that extends from a few cycles persecond to many thousand cycles per second. A piezoelectric accelerometerof the type which may be employed to measure such accelerations isshown, for example, in Patent No. 2,714,672. Other types ofpiezoelectric accelerometers that are suitable for such use are wellknown.

In a piezoelectric accelerometer, a piezoelectric element having twoopposite parallel faces is mounted with one face firmly secured to ahousing that is placed on an object under investigation and with theother face in contact with a mass, or inertial, member. When the objectvibrates, the mass member tends to remain stationary, thus alternatelycompressing and expanding the piezoelectric element between the massmember and the housing. In this action, electric charges are developedon the opposite faces, thus causing electric voltages to be generatedacross the opposite faces in accordance with the acceleration.

In order to detect, measure, and record the acceleration, thepiezoelectric element of the accelerometer is frequently connectedacross the input of an amplifier. For example, in order to make itpossible to measure the amplitude of accelerations over the range offrequencies with an error no greater than 5% in that range, it isnecessary for the product of the capacitance (C) of the accelerometer inmicrofarad (pf.) and the input resistance (R) of the amplifier inmegohms to have a high value of the order of 3 times the period of thelowest frequency components to be detected.

A piezoelectric accelerometer inherently possesses a low capacitancesuch as 500 pf. For this reason, in order to detect signals havingfrequencies down to a low cut-off frequency (l/RC) such as 6 c.p.s., itis necessary for the input resistance of the amplifier to have a veryhigh value, such as 50 megohms. In such a case substantially uniformresponse is customarily obtained down to only about 100 c.p.s. The useof amplifiers having such a high input resistance in such a circuit isfraught with many difficulties. For one thing, it is usually verydifficult to maintain such a high input resistance for any great lengthof time,

ice

especially if the measuring system is usedA uri'der'A a wide" variety ofambient conditions rather than undef highly controlled laboratoryconditions. For example, the input resistance of such an amplifier maybe reduced considerably by virtue of deposits of dust on the partsacross which the input terminals are connected. Furthermore, the inputresistance may drop considerably where the humidity is high. This isespecially true where an amplifier may be exposed to salt air near anocean. In such a case, the resultant reduction of the resistanceintroduces a loss in sensitivity at low frequencies. For example, if thevalue of the input resistance drops ten-fold, the cutoff frequency risesten-fold. Such a change destroys the efcacy of the measuring system atlow frequencies.

Another diflicutly involved in the use of such a system resides in thefact that it is often desirable to connect a piezoelectric transducer toan amplifier located at a remote point by means of a cable that has alength which may vary in length by several hundred feet or more from oneinstallation to another. As a result, the shunt capacitance of the cablemay also vary by a great amount from one installation to another, thusaffecting a great change in the cut-off frequency and a great loss ofsignal strength at all frequencies.

According to the present invention, the foregoing difficulties areovercome by employing an amplifier having a field effect transistor atits input and utilizing a capacitive negative feed-back circuit torender the input impedance of the amplifier capacitive over the range offrequencies of the signal components that are of interest.

Since the piezoelectric transducer with which the amplifier is used isintended to detect and measure acceleration and shock, the amplifiersthemselves may introduce random disturbances caused by the motion whichare to be detected and measured. In order to avoid erratic indicationscaused by vibration and shock, I employ a transistor of thediffused-junction type which has been hermetically sealed in dry gas ina container which is free of any loose solids such as dessicating powderor other powders employed for stabilizing the characteristics of thetransistor.

In the best mode of practicing this invention now known, the inputcapacitance of the amplifier is made large compared with any changesthat are likely to be encountered in the input capacitance due to theuse of cables of different lengths, and the amplifier input capacitanceis very large compared with the capacitance of the piezoelectrictransducer itself and any cable that is expected to be used. Since theamplifier of this invention has -a high input resistance as well as alarge input capacitance and such a special field effect transistor atits input, uniform response is obtained to very low frequencies.

In my prior application, Ser. No. 810,733, a system has been disclosedthat overcomes some of the foregoing difficulties but not as effectivelyas the system of the present application. My prior application disclosesa system that employs a transistorized amplifier which utilizescapactive negative feed-back to render the input impedance oftheamplifier capacitive over a wide range of frequencies of the signalcomponents that are of interest. But the specific amplifier disclosed inthat application is of' the type that has a low input resistance, thoughit has a large input capacitance.

The present invention represents an improvement in L the systemdisclosed and claimed in my prior application,

Ser. No. 810,733. This improvement makes it possible to provide a systemwhich has a uniform response to even lower frequencies than thatemploying the transistorized amplifier disclosed in that priorapplication.

The foregoing and other advantages and features of this invention willhe understood by reference to the following description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a diagram of a system embodying this invention;

FIG. 2 is a graph of amplitude-versns-frequency char acteristics of partof `the system;

FIG. 3 is a perspective view of la field effect transistor of a typeemployed in the practice of this invention;

FIG. 4 is a cross-sectional view taken on the line 4-4 of FIG. 3;

FIG. 5 is a schematic diagram of such a field effect transistor;

FIG. 6 is a schematic cross-sectional view of a field effect transistorunit;

FIG. 7 is a plan view of a part of that unit;

FIG. 8 is a schematic diagram employed in explaining the operation ofsuch a transistor;

FIG. 9 is a graph used in explaining the oper-ation of such atransistor; and

FIG. ly is a perspective drawing of an amplifier of this invention.

Referring to the drawings and more particularly to FIG. 1, there isillustrated a system embodying this invention and employing an inputamplifier 10 having a piezoelectric accelerometer P connected to itsinput 12 -by means of a coaxial cable 14 and having an output amplifierL connected to its output and feeding a utilization unit U such as arecording oscillograph.

The piezoelectric -accelerometer P employs a piezoelectric element thathas two fiat electrodes D1 and D2 on opposite parallel faces thereof andin electrical communication therewith. The piezoelectric element E maybe of any kind generally employed in accelerometers, such as bariumtitanate (BaTiO4) elements or Rochelle or quartz crystals. A spring Ncompressed between the mass M and the wall of the housing H firmly holdsone electrode D1 between the lower face of the piezoelectric element Eand the accelerometer housing H, and the other electrode D2 between theupper face of the element E and an inertial mass M.

In such a piezoelectric accelerometer P, electric charges Q aredeveloped at the two electrodes D1 and D2 in response to compression orexpansion of the crystal when the object O upon which the piezoelectricaccelerometer is mounted is subjected to acceleration. The two chargesdeveloped at the respective electrodes are of equal amounts, but ofopposite polarity. The magnitude of the strain S produced by suchacceleration is proportional to the magnitude of the acceleration, andthe magnitude of the char-ge Q developed is proportional to the strain.Thus for any given piezoelectric transducer Q=KS (l) where K is aconstant. If no external circuits are connected across the electrodes D1and D2, the voltage across the electrode is given by the formula whereCa=capacitance between the electrodes D1 and D2. The cable 14, whichincludes two conductors 14a and 14b, is characterized -by a shuntimpedance between its conductors that is capacitive under the conditionsof operation considered here. The effective shunt capacitance of 'thecable is represented by the lumped element Cc of FIG. '1. In the absenceof the amplifier 10, the effective source capacitance across the outputof the cable is C,=C{Cc (3) For this reason, the voltage actuallyappearing at the end of the cable and available for application to anamplifier,-

ybut prior to connection to the amplifier, is given by the 'followingequation:

El Ci It is thus seen that the voltage available for application to theamplifier depends upon the shunt capacitance of the cable and hence onthe length of the cable as well as on the capacitance of theaccelerometer I. A typical value for the capacitance of a cable is 30pf./ ft. Clearly, Awhen employing a typical accelerometer which may havea capacitance of pf. to 1G00 pf., serious errors may arise where cablesof different lengths are used. These difficulties are obviated by theuse of a transistor amplifier 10 that as an input impedance which issubstantially capacitive and which is high `compared with thecapacitance of the transducer and cable together.

The input amplifier 10 has two input terminals I1 and I2 and two outputterminals O1 and O2. A transistor T1 which is of the diffused-junction,field effect type is contected at the input. This transistor isconnected as a source follower. Additionally, the amplifier employsthree ordinary transistors T2, T3, and T4 which are arranged to amplifyalternating current signals supplied at the output of the field effecttransistor T1 in response to charges generated in the piezoelectricaccelerometer P. The transistors T2 and T4 are of the npn type while thetransistor T3 is of the pnp type and they are connected withcomplementary symmetry. Electric power from a -low voltage source (notshown) is employed to energize the transistors, the positive terminal ofthe low voltage source being connected to the B-iterminal and thenegative end of the voltage source being connected to the grounded B-terminal indicated in FIG. 1.

A feed-back capacitor C0 is connected between the output terminal O1 andthe input terminal I1 to provide negative feed-back. In addition, lowpass filter F is connected between the output terminal O1 and the inputterminal I1 to provide negative feed-back at very low frequencies and atDC. This circuit stabilizes the operation of the amplifier and, inparticular, minimizes long period drift in the characteristics of thefield effect transistor. The two feed-back circuits cooperate to extendto very low `frequencies the range of frequencies in which theamplitude-versus-frequency response is nearly uniform.

The field effect transistor includes a grid or gate electrode GE, -adrain electrode DE, and a source electrode SE. The grid electrode GE isconnected directly to the input terminal I1. The drain electrode DE isconnected directly to the positive terminal B-lwhile the sourceelectrode SE is connected to the negative terminal B- through asource-follower resistor R1. The source electrode SE is also connecteddirectly to the base b2 of the transistor T2 while the collector c2 ofthe second transistor T2 is connected to the B+ terminal through a loadresistor R2. The emitter e2 of this transistor is connected to theground terminal GT through a parallel network consisting of a Zenerdiode Z and a shunt capacitor C2. A -bleeder resistor R21 is connectedin series with the Zener diode Z between the voltage terminals B-land B-in order to establish the voltage of the Zener diode Z at an operatingpoint such that the voltage thereacross is substantially constantthroughout the range of operation of the amplifier.

In practice, the transistor T2 operates at very low current so that itsbase-to-emitter resistance is very high thus enabling voltage changes'applied to the grid electrode GE to appear across the base b2 andemitter e2 of the transistor T2. This voltage is amplified by thetransistor T2 and the amplified voltage is applied to the base b3 of thetransistor T3 which is connected to the collector c2 of the transistorT2. The emitter e3 of the transistor T3 is connected to the B+ terminalwhile the collector c3 of the transistor T3 is connected through a loadresistor R3 to the B- terminal. The output of the transistor T3 whichappears at the collector c3 is applied through a resistor R34 to thebase b4 of the transistor T4. The emitter e4 of this transistor isconnected to the B terminal while the collector c4 is connected throughan output load resistor R4 to the B+ terminal. The amplified signalappearing at the collector c4 of the transistor T4 appears across theoutput terminals O1 and O2.

A Iresistor R31 and -a capacitor C31 are connected in series between thecollector c3 and the base b3 ofthe transistor T3 in order to preventoscillation of the amplifier at high frequencies. Similarly, a resistorR11 and a capacitor C41 are connected in series with the resistor R34between the collector c4 and the base b4 of the transistor T1 for thesame purpose.

The stabilizing filter F employs two series resistors R5 and R3connected directly between the output terminal O1 and the grid elementGE of the field effect transistor T1. It also includes a resistor R7 anda capacitor C7 connected in a shunt arm between the ground terminal GTand the junction J `between the series resistors R5 and R6.

In the operation of this system a charge generated by 'the accelerometerP provides a voltage that is impressed across the input terminals I1 andI2 of the amplifier 10. In effect, this voltage is applied between thegrid element GE and the ground terminal GT and the output of the fieldeffect transistor T1 is applied across the effective impedance providedby the base-to-emitter resistance of the transistor T2 and the parallelimpedance provided by the Zener diode Z and the shunting capacitance C2.The latter impedance is very small and may be neglected in the frequencyrange of operation. The effective resistance between the grid electrodeGE and the source electrode SE of the field effect transistor T1 isexceedingly high. With this arrangement, the voltage impressed upon theinput terminals I1 and I2 is reproduced between the base b2 of thetransistor T2 and the lower input terminal I2. By employing the fieldeffect transistor T1 in the source-follower `mode an impedance matchingeffect is achieved in that the voltage applied across thehigh-resistance input is then reproduced across the low resistancethatis effectively present between the base and emitter of thetransistor T2. This results in power amplification which is useful inconnection with the operation of the amplifier provided by .the three`transistors T2, T3, and T1 and their interconnections.

It can be shown that the total effective capacitance at the input of theamplifier is -where C3=capacitance of the feedback capacitor C0A=amplification of the input amplifier 10 without the feedback capacitorC connected.

Likewise it can be shown that the effective input resistance lookinginto the amplifier 10 is given by the formula in the frequency range inwhich the reactance of the capacitor C7 is negligible where R=theresistance of the series resistor R5 that is connected to the outputterminal O1 `R3=the resistance of the series resistor R3 that is conthefield effect transistor T1 does not appear in Equation 6. The reason forthis is that itis normally very much mw=megohms pf. :microfarads pf.:picofarads In practice it is found desirable to select a field effecttransistor, a source follower resistor R1, and a Zener diode Z whichhave characteristics suitably matched to provide satisfactory operation.To this end for example, when a Zener diode having a characteristicbreakdown voltage of 7 volts is employed, the value of the resistor R1is so selected in relationship to the current fiowing through the sourceelectrode SE as to establish a normal voltage of about 7.5 volts on thebase b2 of the transistor T2. Usually, transistors are selected whichare characterized by source electrode currents in such a range that theresistor R1 is between about 20 kw and 100 kw. 'Transistors are alsoselected in which the minimum drain-to-gate breakdown voltage is 20volts, and the total gate leakage is about 0.3 nanoampere, thetransconductance at about 5K c.p.s. is between 350 and 1000 micromhosand the pinch-off voltage is less than about 12 volts at about 20 C. Inaddition, a field effect transistor is selected for which the .totalgate current is less than nanoamperes at 125 C. For optimum results,field effect transistors producing low noise of electronic origin arechosen. With field effect transistors and source follower resistors R1and Zener diodes Z selected in this way, uniform response can beobtained from about 1 c.p.s to about 10,000 c.p.s. and above over a widetemperature range.

With a piezoelectric accelerometer and cable having a capacitance ofC1=550 pf. and with such an input transistor operating at a point whereits grid-to-source resistance RGS is about 50 megohms or higher, theValues of the effective input capacitance and input resistance of thesystem are as follows:

The amplitude-versus-frequency characteristic of such a system includingaccelerometer P and amplifier 10 is indicated in graph G1 of FIG. 2.Here, it will be noted that the charge-to-Voltage transducer provided bythe piezoelectric accelerometer P and input amplifier 10` has a sha-rpcutoff below about l c.p.s.

The output amplifier L is a DC amplifier which is coupled to the outputof the input amplifier 1t)` through a coupling capacitor C3. In aparticular arrangement in which the capacitance of the couplingcapacitor C3 was nf. and the resistance looking into the outputamplifier L was 2.5 kw, the overall amplitude-versus-frequencycharacteristic introduced by the output amplifier L and the capacitor C3was as shown in graph G2. The c-ombined effect of the two amplifiers issuch that the response of the system is substantially uniform (that iswithin about i2%) down to about 1.5 c.p.s. This result is achievedpartly by virtue of the fact that the amplitude-versus-frequencycharacteristic for the input amplifier rises slightly below about c.p.s.thus compensating for the slight attenuation introduced by the outputamplifier L and coupling capacitor C5 between 5 c.p.s. and 1 c.p.s. Thisslight rise is achieved by suitable proportioning of the values of theimpedances in the DC feedback circuit F in relationship to the value ofthe feedback capacitor C0 and the effective input resistance of theinput amplifier in the absence of feedback.

In order to make it possible to provide a charge-to-voltage amplifierhaving a uniform amplitude-versus-frequency characteristic to such avery low cutoff frequency and which is highly stable in operation andwhich isalso free of disturbances that might otherwise arise because ofthe fact that the motions that affect the accelerometer P may alsoaffect the amplifier 10, a field effect transistor of a special type isemployed. More particularly the field effect transistor is of thediffused-junction type which has been passivated and which has beensealed against exposure to the atmosphere, being sealed in a containerthat is free of any loose particles. Such a transistor is of the generaltype indicated in FIGS. 3, 4, 5, 6, and 7. While the transistor T1identified above is not precisely of the same construction, neverthelessit is of the same general type.

As indicated in FIG. 3, the field effect transistor comprises a solidbody member 100 composed of p-type semiconductive material such assilicon doped with an appropriate microscopic quantity of dopingmaterial such as boron, aluminum, gallium, or indium. A thin layer 120of n-type material is formed by diffusion to form a channel element. Theupper side of this layer 120 is filled with a thin layer of p-typesemiconductive material diffused into place to form a grid element. Thechannel of n-type semi-conductive material is formed by first diffusinga doping material such as phosphorus, arsenic, or antimony into theupper surface of the body 100 in a generally rectangular area. The thinlayer 130 of p-type material is formed by diffusing doping material suchas boron, aluminum, gallium, or indium into the upper side of the layer120 within the boundaries thereof as indicated in FIG. 3. An insulatingcoating is then formed on the upper surface by passivation. Such apassivated coating may be formed, for example, by heating the entireunit 90 to an elevated temperature in an atmosphere of water vapor andhydrogen. In this process, some ofthe silicon on the outer portion onthe upper surface of the unit is converted to an oxide analogous toglass. Then the portions of the passivated coating covering the channelmaterial on opposite sides of the layer 130 of p-type material areetched away and these etched areas are coated with gold, aluminum, orsome other suitable material in order to form ohmic resistances 104 towhich the leads 106 are then bonded.

The ohmic contacts 104 formed, for example, of gold or aluminum aredeposited by evaporation at the ends of the U-shaped channel and arearranged to contact the channel 120 but not the underlying body member100 or each other. Aluminum terminal wires 106 are bonded to theseterminals in order to provide connections for the source element SE anddrain element DE. An aluminum wire is also bonded to a layer 130 ofp-type material within the channel over the layer 120 of n-typematerial. This aluminum wire 110 is also bonded to the underlying bodymember 100. The two bodies of p-type material thus form parts of a gridelement GE that lie on opposite sides of the channel provided by thelayer of n-type mate-rial 120. schematically, the arrangement isindicated in FIG, 5.

In the best embodiment of the invention the transistor 90 of FIG. 3 ismounted in a hermetically sealed container and is provided withterminals as indicated in FIGS. 6 and 7. In practice the transistor 90is bonded to a metallic disc 121 centered in a glass base 122. The twoaluminum leads 106 are connected respectively to terminal wires 134 thatextend through the glass base 122 and the disc 121 is connected to analuminum lead 130 also extending through the glass base 122. Anietallic'case 140 is hermetically sealed to the glass base 122. Inpractice the case is mounted in place in a moisture-free atmosphere ofan inert gas or air.

When such a transistor 4is employed the outer surface of the glass base122 is protected against contamination in order to minimize alterationsin the characteristic of the unit that might otherwise be caused by theoxposure of these wires to the atmosphere. To this end the entireamplifier 10 is imbedded, that is potted, within insulating material andthe potted amplifier is placed within a hermetically sealed metal casethrough which mutually insulated `and widely spaced apart terminals O1,B+, and B- project, as illustrated schematically in FIG. 10. Inpractice, the grounded output terminal O2 and the grounded inputterminal I2 are connected to the outer terminal k1 of the coaxialconnector K, which, in turn, is connected to the case and grounded. Alsoin practice, the input terminal I1 (not shown in FIG. 10) is connectedthrough the resistor R0 to the central terminal k2 of the coaxialconnector K, the resistor R0 also being potted in the case.

While many different kinds of field effect transistors are available andare suitable for my purpose, I have found the Amelco FE 202 transistorvery satisfactory. Such a transistor has a characteristic generallyindicated in FIG. 9. Here, abscissae represent source-to-drain voltage,ordinates represent source-to-drain current, and the various graphscorrespond to various parametric values of grid voltages, as indicated,that is, voltages applied between the grid element GE and the drainelement DE. In practice, when a load resistor LR is connected betweenthe drain element and the voltage supply as indicated in FIG. 8, thetransistor operates along a load line indicated by the straight line LLindicated in FIG. 7. However, when operated as a source follower with aresistance between the source and the negative terminal B- smallcompared with the internal resistance of the transistor between the gridelement GE and the source element SE as in FIG. 1, the voltage appliedbetween the grid element and the negative terminal B- is transferreddirectly to the load circuit between the source element SE and theterminal B-.

It is thus seen that -by employing a field effect transistor of the typedescribed in a DC amplifier having a negative feedback circuitconsisting of a capacitive element C0 and another negative feedbackcircuit of the DC type, it is possible to provide a charge-to-voltageamplification which is very nearly uniform to exceedingly lowfrequencies of about l c.p.s. and which is highly stable and free ofnoise of a vibratory origin when in operation. It will be appreciated inview of the foregoing disclosure that the invention may be practiced inmany other forms than that specifically described. It is therefore to beunderstood that many alterations may be made `therein without departingfrom the scope of the invention as defined by the appended claims.

The invention claimed is:

1. In a charge to voltage transducing system:

a piezoelectric transducer inclu-ding a piezoelectric element and havingan output at which a variable charge is generated to provide a sourcesignal in accordance with the m-agnitude and sign of the displacement ofonle part of the piezoelectric element relative to anot er;

an amplifier having an input and an output and including an input stageconnected to receive said source signal from said transducer output,said input stage comprising a field effect transistor having a gridelement, a source element, and a drain element, said grid element beingconnected in said amplifier input for varying the magnitude of thesignal appearing at said amplifier output in accordance with thevariable charge generated by said piezoelectric transducer; and

a negative feedback circuit including a capacitor conthe inputcapacitance of said amplifier with such feedback being very largecompared with the combined capacitance of said transducer and theconnections thereof to said amplifier in said frequency range wherebythe signal appearing at the output of said amplifier is substantiallyproportional to the quantity of charge appearing at the output of saidtransducer over said frequency range the time constant at the input ofsaid amplifier formed by the resistance looking into said field effecttransistor and the total effective capacitance connected thereto havinga high value such that said voltagey transducing system has a cutoff ata very low subaudio frequency and substantially uniform response atfrequencies in a range above said low frequency and up to a very highfrequency.

ing the magnitude of the signal appearing at said amplifier output inaccordance with the variable charge generated by said piezoelectrictransducer; and a negative feedback circuit including a capacitorconnected between said amplifier output and said amplifier input forrendering the impedance looking into said input primarily capacitiveover a predetermined range of frequencies; the input capacitance of saidamplifier with such feedback being very large compared with the combinedcapacitance of said transducer and the connections thereof to saidamplifier in said frequency range whereby the signal appearing at theoutput of said amplifier is substantially proportional to the quantityl5 of charge appearing at the output of said accelerometer over saidfrequency range the time constant at the input of said amplifier formedby the resistance looking into said field effect transistor and thetotal effective capacitance connected .thereto having a high value suchthat said voltage transducing system has a cutoff at a very low subaudiofrequency and substantially uniform response at frequencies in a rangeabove said low frequency and up to a very high 2. In a charge to voltagetransducing system as defined in claim 1 wherein:

said field effect transistor is of the diffused-junction type.

3. In a charge to voltage transducing system as defined frequency. inclaim 1 wherein: 25 5. In a vibration detecting system as defined inclaim 4 an addition-al amplifier is connected to the output of wherein:

said first mentioned amplifier, said additional amplithe time constantat the input of said amplifier formed fier having a substantiallyuniform response in said frequency range.

. In a system for detecting vibration,

piezoelectric accelerometer responsive to vibrations, said accelerometerincluding a piezoelectric element arranged to be subjected to a force inaccordance with by the resistance looking into said field effecttransistor and the total effective capacitance connected thereto has arelatively high value whereby said vibration detecting system has asubstantially uniform response over a frequency range from about 1c.p.s. -to about 10,000 c.p.s.

6. A vibrat-ion detecting system as defined in claim 4 wherein said gridelement is resistively connected to the output of said transducer.

such vibration, said accelerometer developing at its output an electriccharge that varies in accordance with the magnitude and sign of suchforce;

an amplifier havin-g an input connected to the output of saidaccelerometer and having an output, said amplifier having a field effecttransistor connected in its input, said field effect transistor having agrid element, a source element, and a drain element, said grid elementbeing connected in said amplifier input, for vary- No references cited.

MILTON O. HIRSHFIELD, Primary Examinar.

J. D. MILLER, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.5,556,868 Dated December 5, 1967 Inventor(s) ROBERT H. COTHER It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

column l, line 11, "1961;" should be 1965.

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD'M.FLETCHER,JR. ROBERT` GOTTSCHALK Attestlng Offlcer Commissionerof Patents

1. IN A CHARGE TO VOLTAGE TRANSDUCING SYSTEM: A PIECZOELECTIC TRANSDUCERINCLUDING A PIEZOELECTRIC ELEMENT AND HAVING AN OUTPUT AT WHICH AVARIABLE CHARGE IS GENERATED TO PROVIDE A SOURCE SIGNAL IN ACCORDANCEWITH THE MAGNITUDE AND SIGN OF THE DISPLACEMENT OF ONE PART OF THEPIEZOELECTRIC ELEMENT RELATIVE TO ANOTHER; AN AMPLIFIER HAVING AN INPUTAND AN OUTPUT AND INCLUDING AN INPUT STAGE CONNECTED TO RECEIVE SAIDSOURCE SIGNAL FROM SAID TRANSDUCER OUTPUT, SAID INPUT STAGE COMPRISING AFIELD EFFECT TRANSISTOR HAVING A GRID ELEMENT, A SOURCE ELEMENT, AND ADRAIN ELEMENT, SAID GRID ELEMENT BEING CONNECTED IN SAID AMPLIFIER INPUTFOR VARYING THE MAGNITUDE OF THE SIGNAL APPEARING AT SAID AMPLIFIEROUTPUT IN ACCORDANCE WITH THE VARIABLE CHARGE GENERATED BY SAIDPIEZOELECTRIC TRANSDUCER; AND A NEGATIVE FEEDBACK CIRCUIT INCLUDING ACAPACITOR CONNECTED BETWEEN SAID AMPLIFIER OUTPUT AND SAID AMLIFIERINPUT FOR RENDERING THE IMPEDANCE LOOKING INTO SAID INPUT PRIMARILYCAPACITIVE OVER A PREDETERMINED RANGE OF FREQUENCIES; THE INPUTCAPACITANCE OF SAID AMPLIFIER WITH SUCH FEEDBACK BEING VARY LARGECOMPARED WITH THE COMBINED CAPACITANCE OF SAID TRANSDUCER AND THECONNECTIONS THEREOF TO SAID AMPLIFIER IN SAID FREQUENCY RANGE WHEREBYTHE SIGNAL APPEARING AT THE OUTPUT OF SAID AMPLIFIER IS SUBSTANTIALLYPROPORTIONAL TO THE QUANTITY OF CHARGE APPEARING AT THE OUTPUT OF SAIDTRANSDUCER OVER SAID FREQUENCY RANGE THE TIME CONSTANT AT THE INPUT OFSAID AMPLIFIER FORMED BY THE RESISTANCE LOOKING INTO SAID FIELD EFFECTTRANSISTOR AND THE TOTAL EFFECTIVE CAPACITANCE CONNECTED THERETO HAVINGA HIGH VALUE SUCH THAT SAID VOLTAGE TRANSDUCING SYSTEM HAS A CUTOFF AT AVERY LOW SUBAUDIO FREQUENCY AND SUBSTANTIALLY UNIFORM RESPONSE ATFREQUENCIES IN A RANGE ABOVE SAID LOW FREQUENCY AND UP TO A VERY HIGHFREQUENCY.